In all frankness, even the complete loss of L-L aid to the USSR in 1941 would not have prevented Barbarossa from failing; by all accounts the Soviets had already stopped the Germans more or less by themselves during the winter of that year, well before Lend Lease took effect. It would only start to matter beginning in 1943 or so when the Red Army finally turned the tide and began hammering the Germans westward. Even if somehow all foreign aid to Russia was cut off indefinitely the USSR would remain unconquerable for the Reich in much the same way the Nationalist China remained unconquerable for Japan: the inadequacy of the Wehrmacht's logistical chain and the battle of Moscow more than amply demonstrated this.